Design Guidelines for Farm Constructed Wetlands

2.2.4 Design Guidelines for Farm Constructed Wetlands

2.2.4.1 Background and Water Treatment Requirements

Once site assessment and selection has been completed, the detailed design of an FCW can be conducted, taking into account the farm and farmyard structure and management practices. When designing a FCW, the following aspects are there- fore of major importance (Zedler 2003; Harrington et al. 2005):

• objectives behind the construction of the FCW; • characteristics of the farmyard runoff to be treated (volume and quality); • water quality targets to be achieved; and • land availability to achieve the target water quality.

Adequate pretreatment, retention time, management, and operation (e.g., re- moval of sediment and regular inspection) and design for management (e.g., ac- cess) are required to achieve effective water treatment through a FCW. A FCW that is fit for a certain purpose should have the following attributes (Harrington and Ryder 2002; Rice et al. 2002; Scholz et al. 2007a, b):

• be reliable and efficient in water treatment, particularly during storm events, extreme rainfall with increased hydraulic loadings, and also under relatively cold conditions;

• be capable of coping with accidental spillages; • be flexible and versatile; • be relatively simple to build; • have low operation and maintenance requirements and costs; • have low energy consumption; • be a good landscape fit; • enhance habitat and biodiversity (Froneman et al. 2001); and • be safe for farmers and for the public.

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2.2.4.2 Runoff Capture and Conveyance

One of the early steps in FCW construction should be to ensure that any contami- nated runoff, such as from roofs, farmyards, and tracks, be captured properly. Runoff from adjacent land should usually not enter the FCW. However, regional recommendations may vary.

The conveyance of waters to, within, and from the FCW must consider the fol- lowing (Scholz 2006a): collecting water from the farm, conveying that water to the wetland, moving water within the wetland, and moving water out of the wet- land. Where possible, the effluent should flow by gravity to minimize maintenance and energy costs.

It is essential that any containment be secure and that only water with accept- able concentrations of contaminants be discharged to watercourses or groundwa- ter. The FCW embankments retaining the water flowing through the system must

be sufficiently high to allow for the accumulation of sediment and detritus. The soil lining the base must adequately impede infiltration to protect groundwater (Dunne et al. 2005a, b; Keohane et al. 2005; Scholz et al. 2007b).

2.2.4.3 Hydraulics, Water Balance, and Residence Time

The periodic nature of precipitation and the interception and uptake of water by emergent vegetation, evaporation, and ground infiltration has the capacity to arrest water flow between the individual segments of a FCW. This creates a freeboard between the outlet level and the level of the water contained within an individual wetland cell. It also provides each wetland cell with ‘additional’ receiving hydrau- lic capacity before flow to the next segment can resume, thus enhancing the hy- draulic residence time (Dunne et al. 2005a, b; Scholz et al. 2007).

The treatment effectiveness of surface-flow wetland systems in comparison to sub-surface-flow systems (Mantovi et al. 2003) is typically based on having ap- propriate hydraulic residence times, which depend very much on the specific site conditions (Harrington et al. 2005). In shallow, emergent, or vegetated wetlands, such as FCW, this depends on having sufficient functional wetland area with an appropriate length-to-width ratio and a high emergent vegetation density. The hydraulic effectiveness of the FCW can be maximized by the following measures (Scholz et al. 2007):

• segmentation of the wetland into a number of wetland cells of appropriate con- figuration (see below);

• avoidance of preferential flow; • dense vegetation stand; and • managing the water depth to ensure optimal functioning (Scholz 2007).

The velocity of the water flow through the FCW is determined by the volumet- ric flow and the cross-sectional area of the water channel. Minimizing the velocity enhances the settling of suspended solids and promotes a longer contact time with

2.2 Guidelines for Farmyard Runoff Treatment with Wetlands 47

emergent vegetation whose surfaces support biofilms (Scholz et al. 2002; Kanta- wanichkul and Somprasert 2005; Scholz et al. 2007).

Wind and temperature gradients can generate water movement between the dif- ferent aquatic strata within a wetland cell. Emergent vegetation minimizes mixing, thereby allowing the cleaner water to flow preferentially along the surface, espe- cially during periods of large precipitation-generated flow. In the initial receiving wetland cell, floating vegetation may develop (typically Glyceria fluitans and Agrostis stolonifera) and water flow will be partially sub-surface, thus having the additional advantage of reducing odors (Dunne et al. 2005a; Scholz and Lee 2005; Scholz et al. 2007).

2.2.4.4 Wetland Sizing, Inlet, and Outlet

The design and sizing of FCW has often focused on phosphorus, which is recog- nized as one of the most difficult contaminants to remove from water and is a limiting nutrient in many freshwater ecosystems (Braskerud 2002). For example, a catchment-specific study of 13 wetland systems in Waterford (Ireland) showed that, to achieve a mean MRP concentration at the outlet of 1 mg/l or less, the wet- land area required was at least 1.3 times the farmyard area, and that each system should contain approximately four cells.

The design is based on two assumptions: the larger a wetland, the more phos- phorus removal can be expected; and all ICW studied near Waterford (Ireland) were at the designated threshold of failure for phosphorus (1 mg/l MRP for the outflow or near it in cases of no flow (Scholz et al. 2007). This finding relating to MRP is, however, not universally applicable.

The aspect ratio is defined as the mean length of the wetland system divided by the mean width. The study conducted in Ireland showed that to obtain an outlet MRP concentration of 1 mg/l or less, the FCW aspect ratio should be less than 2.2. In fact, the closer the aspect ratio is to 1 (i.e., the more the FCW shape is square or round), the better the wetland treatment (Scholz et al. 2007).

Sizing must also take into account the footprint of the upper embankments, which should be between 2 and 3 m wide to ensure stability and to provide easy access for maintenance and monitoring. For safety reasons, inner embankments should be gently sloping.

Inlets and outlets should be kept as simple as possible and avoid the use of con- crete and overengineered structures. Pipe diameters should be at least 150 mm to avoid clogging. Stone chippings should be placed beneath the inlet and outlet pipes to prevent scouring. Elbow pipes fitted to linear ones can be used to control the water level and the outflow of each cell. Carty et al. (2008) provide further details on sustainable wetland design.

2.2.4.5 Landscape Fit, Biodiversity, and Life Span

The potential visual aspect of the FCW system design is important for achiev- ing empathy from both farm dwellers and the local community. Usually, FCW

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with curvilinear shaping and virtually level embankments have a more ‘natural’ appearance.

Several measures can improve the landscape fit and biodiversity of an FCW. Through BMP on the farm, the level of contaminated water discharging to the ponds can be reduced (Scholz 2006a; Zheng et al. 2006). Wherever possible, the FCW should be located near (but not connected to) existing wetlands, ponds, and lakes to allow for natural colonization by plants and animals. The FCW cells should be irregular in shape, with gently sloping embankments and areas of deeper water, and contain islands where sufficient area is available. The use of locally occurring wetland plant species for establishing habitats and enhancing biodiver- sity appropriate to the locality is also likely to further increase the robustness and sustainability of the system (Froneman et al. 2001; Scholz 2007; Scholz et al. 2007b).

The area surrounding the FCW can be planted with trees and shrubs, but trees are not recommended on the FCW embankments. If possible, small pools around the main system should be created to collect runoff from adjacent fields and create additional aquatic habitat (Froneman et al. 2001; Carty et al. 2008).

Wetland embankment height, inflowing solids, and accumulating detritus de- termine the functional life span of each segment of the FCW. With detritus accu- mulation and a minimum embankment height of 1 m, a life span of between 50 and 100 years is expected. However, the life span can be virtually indefinite if detritus removal takes place regularly, as discussed by Scholz et al. (2007b) and in the section on maintenance below.